Net Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend		$ 607.0	$ 615.3
Less Investment Expenses		9.0	9.6
Less Investment Income on PFA Assets		3.3	3.4
Net Investment Income		594.7	602.3
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend		544.9	557.1
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend		17.9	15.4
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend		26.7	27.3
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend		4.7	4.4
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	[1]	4.1	(0.2)
Partnership Interest
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	[2]	2.2	5.6
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend		(0.5)	2.1
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend		$ 7.0	$ 3.6

[1] 1 The net unrealized gain (loss) recognized in net investment income for the first three months of 2019 and 2018 related to equity securities still held at the end of reporting period was $3.1 million and ($2.0) million, respectively.

[2] The net unrealized loss recognized in net investment income for the first three months of 2019 and 2018 related to private equity partnerships still held at the end of reporting period was $2.7 million and $1.6 million, respectively.